1. Field of the Invention
This invention relates generally to digital video technology and more particularly to algorithms for smoothing artificial discontinuities between adjacent image blocks, generated by low bit-rate video coding, without introducing undesired blur.
2. Description of the Related Art
Today's low-bit-rate video coding standards, such as MPEG-4, ITU-T H.263, etc. contain algorithms that enable a variety of applications, such as video conferencing and video phones. These standards, and the systems that use them, take advantage of temporal redundancy as well as spatial redundancy to compress the video data. While these standards are quite effective in many ways, the standards sometimes generate decompressed images that exhibit artificial discontinuities between image blocks, also referred to as blocking artifacts. These blocking artifacts are caused primarily by quantization during the quantization step of the compression process.
FIG. 1 is a simplified schematic that pictorially represents a blocking artifact associated with image data. Here, a frame of data fn, includes boundary 102 located between block 1 and block 2 of the frame of image data. Block 1 includes pixel a and pixel b, while block 2 includes pixel c and pixel d. Line 100 denotes the pixel values for pixels c and d relative to pixels a and b. For example, line 100 may represent that pixels c and d correspond to a value of 1, while pixels a and b correspond to a value of 0. Accordingly, the decoded video will have a blocky effect at boundary 102 between block 1 and 2. FIG. 2 is a graphical representation of the original intensities compared to the distortion of the original intensities due to a blocky effect. The original intensities display a gradual and smooth increase across boundary 102 rather than an abrupt transition. However, due to the truncation of the high frequency coefficients by quantization, a blocky effect is observed in the decompressed image.
In block-based coding, monotone areas of the original image, where the pixel intensity changes gradually, suffer most noticeably from the abrupt changes across the block boundary, leading to blocking artifacts. In terms of discrete cosine transform (DCT), when the DCT coefficient quantization step size is above the threshold for visibility, discontinuities in grayscale values are caused by the removal of AC coefficients due to quantization. These discontinuities become clearly visible at the boundaries between blocks of a frame of the video image.
Various deblocking schemes have been proposed in still image coding as well as video coding, where most of the deblocking schemes use low pass filters in the spatial domain. A well-known method for reducing blocking artifacts is based on the theory of alternative projection onto convex sets (POCS), under the assumption that blocking artifacts are always located at block boundaries. However, this method is only applicable to still images because of an iteration structure and long convergence time.
In video coding, in order to maintain a specified bit rate, a proper quantization of the transformed coefficients must be performed. As a result of the quantization, the blocky effect appears in the reconstructed images. This artifact can be strongly visible and as such, severely degrades the image quality. One attempt to improve the image quality is to apply post-processing steps to the decoded video data, such as low pass filters applied to the spatial domain. However, one short coming with current post-processing steps is their computational complexity, which requires about 30-40% of the total computational power needed in the receiver. It should be appreciated that this type of power drain is unacceptably high for mobile terminals, i.e., battery enabled consumer electronics, such as terminals incorporating thin film transistors (TFT) technology, super-twisted nematic (STN), and mobile digital-thin film diode (MD-TFD). Another shortcoming of the low pass filters currently being used is that the amount of time for the filtering operation may cause a noticeable delay in the presentation of the image. This delay is especially noticeable with respect to portable electronic computing systems due to the limited resources of the embedded systems controlling these devices.
As a result, there is a need to solve the problems of the prior art and to provide a method and apparatus for enabling a post-processing algorithm for real-time applications that reduces the blocky artifact more efficiently from both a power and a time standpoint.